The principal investigator recently developed a new ion-selective electrode (ISE) which responds to the activity of dibasic phosphate. This phosphate- ISE has been shown to have good selectivity for phosphate ions over biologically important anions such as chloride, lactate and acetate. The principal investigator now proposes to use this phosphatase-ISE to study phosphate binding, mineral saturation, and the role of phosphatespecies in saliva and dental plaque fluid. To accomplish these goals, the phosphate- ISE will be employed for the following studies; (1) directly determine binding constants for phosphate ion pairs with biologically relevant cations in well-defined chemical systems. Binding constatns determined through the direct measurement of phosphate activities will be used to verify and possibly improve the accuracy of published binding constants; (2) directly determine the activity of phosphate species in human saliva and dental plaque fluid. The activities of phosphate species, Ca2+, Mg2+, H+, and F- determined simultaneously by ISE methods in these physiological fluids will be used to calculate ionic activity products (IAP) with respect to various salts; (3) develop a micron-sized phosphate-ISE for the measurement of phosphate activities in micro-samples. Caries is a site- specific disease where the amount of plaque at that site is small. A micron-sized version of the phosphate-ISE will be developed to assist in the analysis of these single-site samples; (40 determine phosphate-ISE selectivity coefficients for anions that are important for physiological experiments or biochemical systems. ISEs often suffer from interferences that limit the utility of the electrode.Selectivity coefficients for physiological buffers such as HEPES. CAPS, etc., and biological anions such as fatty acids, organic acids, etc., will be essential for the successful application of the phosphate-ISEf or biochemical analysis; and (5) study the mechanism for phosphate-ISE selectivity to phosphate- ions over other ions, in conjunction with computer modeling and NMR. The results from this mechanism study may lead to an improved phosphate-ISE.